Accurate determination of signal timing is desirable in a wide variety of communication and navigation applications where precise, reliable signal reception is important. For example, state-of-the-art position location and communication systems can provide accurate, reliable three-dimensional position determination of a handheld or portable, spread spectrum communication device within milliseconds without interruption of voice or data communications. Among techniques employed to determine the position of a mobile communication device is the reception at the mobile communication device of multiple timing signals respectively transmitted from multiple transmitters at different, known locations. By determining the range or pseudo-range to each transmitter from the arrival time of the timing signals, the mobile communication device can compute its position using trilateration.
One example of a position-determination system involves a two-way round-trip ranging message scheme. A mobile communication device whose position is to be determined transmits outbound ranging messages to a group of reference transceivers. The reference transceivers, whose positions are known, respond by transmitting reply ranging messages. Upon reception of each reply ranging message, the mobile communication device determines the round-trip signal propagation time from which the range can be determined by subtracting the far-end turn-around time and internal processing delays from the elapsed time between transmission of the outbound ranging message and receipt of the corresponding reply ranging message. This round-trip approach avoids the need to synchronize the mobile communication device with the reference transceivers, since the relevant measurements are made in the same time frame of the mobile communication device.
Another example of a position-determination system is the RF ranging function built into the architecture of wireless communication systems such as those of Verizon Wireless or AT&T in the U.S. These systems take advantage of their infrastructure to generate RF ranging signals used for location determination. For example, the CDMA-based digital cellular standard IS-95 uses pilot tones for location determination. The synchronized pilot tones are sent from base stations and received by mobile handsets. One-way distance measurements are made by a particular handset to several base stations, and the data are sent to a central location where the trilateration calculation is made.
When measuring the range to an object or another device, a precise determination of the signal propagation time between the devices must be made. The signal propagation time can be derived by knowing the transmission and reception times of one or more ranging signals traveling along a direct path between the devices. The accuracy of the position determined by these systems depends largely on the accuracy with which the receiving devices can determine the time of arrival of the ranging signals traveling along a direct path between the devices. The IS-95 standard uses a one-way technique where pilot signals are transmitted by base stations and received by the mobile units being located. Time of arrival is determined by interpolating correlator output samples. The peak sample, one pre-peak sample, and one post-peak sample are used in the interpolation calculation to estimate the timing of the true peak of the correlation function. The timing of the peak of the correlation function corresponds to the time of arrival of the signal. The sample rate at the output of the correlator, 2.5 Msps, is twice the chip rate of 1.25 Mcps. However, use of the post-peak sample in the interpolation calculation increases susceptibility to multipath interference. Another shortcoming of this approach is the use of a standard PN sequence in the ranging waveform. This will result in a correlation function with high sidelobes and poor performance in multipath environments. Accordingly, there remains a need for improved techniques for accurately determining the time of arrival of signals in position-determination systems.